Ceramic honeycomb structures are widely used as anti-pollutant devices in the exhaust systems of automotive vehicles, both as catalytic converter substrates in automobiles, and diesel particulate filters in diesel-powered vehicles. In both applications, the ceramic honeycomb structures are formed from a matrix of thin ceramic webs which define a plurality of parallel, gas conducting channels. To reduce the pressure drop that the exhaust gases create when flowing through the honeycomb structure, the web walls are rendered quite thin, i.e. on the order 2-30 mils, depending upon whether the structures are to be used a catalytic converters or diesel particulate filters. In either case, the matrix of cells is surrounded by an outer skin which may be also quite thin.
In the first steps of manufacturing such substrates, generally the ceramic-forming ingredients are mixed together with a binder and liquid vehicle to form a paste-like substance which is extruded into a green body honeycomb “log.” These green body logs are next conveyed through a drying station where they are subjected to microwaves, radio-frequency waves or induction currents to set or gel the binder. The log-like honeycomb extrusion may then be cut into segments along its longitudinal axis to form individual green body honeycomb structures, which are then loaded into a kiln. The honeycomb structures are fired at temperatures of typically 1300° C. or higher in order to sinter the batch constituent particles present in the extruded material into a fired ceramic honeycomb structure. The resulting fired ceramic honeycomb structures may then be subjected to a number of other manufacturing steps (such as plugging, washcoating, further firing steps, and packaging) before being rendered into a final product.
Due to the thinness of the outer skin and the inner cell-forming webs, the honeycomb structures may be relatively fragile and subject to damage. This is particularly true in the first steps of manufacture, when the web matrix and outer skin is in a green body state, being formed from a dried “clay” of unfused, particulate ceramic-forming ingredients held together by an organic binder. However, certain irregularities can also occur to the substrates during subsequent manufacturing steps from the thermal stresses that the unfinished ceramic structures may undergo during the firing process, and the necessary subsequent mechanical handling of the fired bodies as they are converted into finished products. Such irregularities in the structures may take the form of internal cracks and voids, chips and dents, and separations between the outer skin and the inner matrix of webs.
To reduce the occurrence of such irregularities, it would be desirable to have a quality control procedure which allowed the manufacturer to reliably trace any defective ceramic honeycomb structure back to the specific factory, extruder, dryer, kiln, and batch ingredients that it originated from. Such a procedure would allow the manufacturer to review the particular manufacturing parameters used to fabricate the honeycomb structure and to modify its manufacturing operation in order to reduce the occurrence of such irregularities in future articles. Accordingly, it is a known procedure to mark, after the final firing or heating step, finished ceramic honeycomb structures with marks containing manufacturing information so that remedial manufacturing operations may be implemented in the event of irregularities.
Unfortunately, the applicants have observed that such a marking procedure does not reliably result in an accurate recovery of the manufacturing information associated with a particular ceramic honeycomb structure. In particular, the applicants have observed that subsequent to the manufacture of the green bodies of such structures, different batches of ceramic structures come from different kilns necessarily become mixed together in order to efficiently implement other stages of the fabrication process. Additionally, different unfinished ceramic structures may be removed from one or more manufacturing loops, put into storage, and then later reintroduced into another manufacturing loop. Hence a quality control process where manufacturing information is printed on the finished ceramic honeycomb structures may not accurately reflect the actual manufacturing conditions and history of the structures, as structures which end up adjacent to one another in the final stages of manufacturing might have quite different manufacturing histories.